The background of this invention is found in U.S. Pat. No. 4,646,541 (hereinafter the Prior Patent) which discloses the general subject matter of this invention. This invention therefore should be considered with reference to this Prior Patent which includes the common inventors F. Bert Cook and Edward A. Reid, Jr. and is assigned to the same assignee as this invention.
U.S. Pat. Nos. 4,742,693, 4,719,767, 4,691,532, 4,742,687, and 4,722,193 are sibling patents of the Prior Patent and are pertinent to the disclosure of this invention providing further background information on this subject matter.
In the quest for improvement in Absorption Refrigeration and Heat Pump Systems, the common measure of performance is the often referred to "coefficient of performance", i.e., COP. As used herein, coefficient of performance, i.e, COP, is defined as the energy transferred at the load in a unit of time divided by the energy provided to the system in the same unit of time which is well understood by those skilled in the art. Other measures of performance include reduction in complexity; or stated conversely, apparatus and system simplification.
Absorption systems are usually very efficient during the heating cycle, when a source of heat, such as a natural gas flame is used. On the other hand, these systems are less efficient in the cooling cycle.
Air cooled refrigeration circuits of the mechanical vapor compression type have also been demonstrated which can be reversed to provide either heating or cooling to a load by switching the flow of an intermediate heat transfer solution typically consisting of water and antifreeze solutions such as ethylene glycol, etc.
Liquid cooled absorption refrigeration circuits using the double effect generator cycle to achieve high efficiency are commercially available. However, these systems using water as the refrigerant are not suitable for use in heating a conditioned space (the heating load) since the refrigerant freezes at 32.degree. F. and therefore cannot be used in a space heating system at ambient outside temperatures below approximately 40.degree. F.
Absorption refrigeration and heat pump systems are well known in their basic operating characteristics and need little further description except to establish the definitions and context in which this invention will be later described.
In a typical system a refrigerant, water or other phase change material is dissolved in an absorbent (typically lithium bromide or other salts) and these are often called the "solution pair". The refrigerant is absorbed or desorbed (expelled) in or out of solution with the absorbent to varying degrees throughout the system and the heat of absorption is added or extracted to produce heating and cooling effects.
The solution pair enters a generator where it is subjected to heat and the applied heat desorbs (expels) a portion of the refrigerant in the form of a vapor which is conveyed to the condenser. There, external cooling condenses the refrigerant vapor to liquid, which is conveyed through an expansion valve, into an evaporator where heat is gained. In the refrigeration system operation the heat gained in the evaporator is from the cooling load.
The low pressure vapor then passes to an absorber where cooling allows the absorbent solution to absorb the refrigerant vapor. The solution is then conveyed to a recuperator by a pump. The recuperator is a counterflow heat exchanger where heat from the absorbent/refrigerant solution flowing from the generator to the absorber, heats the returning solution pair flowing from the absorber to the generator. In the heating cycle, the cooling applied at the absorber and/or the condenser is the heat delivered to the heating load.
As a matter of convenience and terminology herein, each part of the absorption system which operates at the same pressure is termed a chamber.
Conventional absorption refrigeration/heating systems are two chamber systems although three chamber systems appear in the prior art and have seen limited use. When operated as heat pumps, two chamber systems give respectable heating performance but give poor cooling performance.
Using ammonia (NH.sub.3) as the refrigerant and water (H.sub.2 O) as the sorbent, heat pumping can occur from an ambient air source which is at temperatures below freezing. Where the air is treated as if it were dry so that no defrosting is necessary, the typical two chamber NH.sub.3 /H.sub.2 O heat pump would represent a significant improvement over what would be expected of a simple furnace. However, since heat pumps are more expensive than furnaces, cooling season performance benefits are needed to justify the added expense. In other words, the heat pump must act as an air conditioner also to offset the additional cost of the heat pump combined with separate installation of an air conditioner with a furnace.
For cooling, an NH.sub.3 /H.sub.2 O system is predicted to have a COP equal to about 0.5. This low performance index causes unreasonable fuel (or energy) costs from excessive fuel (or energy) use.
Three-chamber systems of various types have been suggested which would improve the performance by staging the desorption process into effects. This allows for increasing the actual temperature at which the driving heat is added to the system (cycle). Until the invention of U.S. Pat. No. 4,646,541 it was thought that this increase in temperature would represent an unreasonably high pressure, especially from ammonia/water systems, and would force the system to operate in regions for which data is not readily available.
In addition the pressure has tended to rule out ammonia/water in a three-chamber system. The search for organic materials such as halogenated hydrocarbons and other refrigerants as a replacement for the ammonia has been limited by fluid stability at these higher temperatures. Normal organic refrigerant stability tests have indicated that it is necessary for oil to be present for operation in vapor compression refrigeration systems. These high operating temperatures rule out most of the common refrigerants, particularly from being heated directly by combustion products which often cause local hot spots, which result in working fluid degradation and/or corrosion of components.
The heat actuated, air cooled, double effect generator cycle absorption refrigeration system of Prior Patent (U.S. Pat. No. 4,646,541) and the sibling patents therefrom overcome limitations of the existing prior art technology. The air cooled system therein eliminated the need for cooling water and the use of ammonia as the refrigerant avoids refrigerant freezing during heating operation. The double effect generator cycle permits high efficiency through internal heat recovery in the absorption refrigeration circuit. The use of sodium thiocyanate as the absorbent eliminates the need for analyzers and rectifiers to purify the refrigerant stream with the resultant loss of unrecoverable heat.
This invention is directed to further improvements and simplifications of the above described prior patented system. It applies to an integrated three-chamber system having one solution pair using a material of unusual fluid stability at higher temperatures when manipulated in an apparatus and system to take advantage of its properties. The typical preferred solution pair for operation as part of the system and components of this invention is ammonia as the refrigerant and sodium thiocyanate as the absorbent.